1. Field of the Invention
The present invention relates to a reference frequency signal switching circuit and, more particularly, to a reference frequency signal switching circuit capable of selectively outputting an internal reference frequency signal generated by an internal oscillator or an external reference frequency signal supplied from the outside and maintaining the output at a constant level irrespective of the level of the external reference frequency signal supplied.
2. Description of the Related Art
Hitherto, in transmitter of a cable television (CATV), a reference frequency signal switching circuit for supplying a reference frequency signal to operate the transmitter is used.
The reference frequency signal switching circuit has therein an internal oscillator for generating an internal reference frequency signal and operates so as to selectively output an internal reference frequency signal outputted from the internal oscillator or an external reference frequency signal supplied from the outside. In the case of operating a single transmitter, the internal reference frequency signal outputted from the internal oscillator is used. In the case of using a plurality of transmitters in the same system, in order to operate the transmitters at the same frequency, the external reference frequency signal supplied from the outside is used.
FIG. 5 is a diagram showing an example of the construction of a conventional reference frequency signal switching circuit.
As shown in FIG. 5, the conventional reference frequency signal switching circuit comprises: an external reference frequency signal input terminal 31 to which the external reference frequency signal is supplied; an input circuit 32; an internal oscillator 33 for generating an internal reference frequency signal; a buffer amplifier 34; a signal switching circuit 35; a reference frequency signal output terminal 36; a high voltage power source terminal 37; and a low voltage power source terminal 38. The signal switching circuit 35 has a tuned amplifier 35.sub.1 a detecting circuit 35.sub.2, a smoothing circuit 35.sub.3, a first switching transistor 35.sub.4, a second switching transistor 35.sub.5, a first switching diode 35.sub.6, a second switching diode 35.sub.7, a coupling capacitor 35.sub.8, a collector resistor 35.sub.9, an inductor 35.sub.10 and a bias resistor 35.sub.11.
The external reference frequency signal supplied to the external reference frequency signal input terminal 31 passes through the input circuit 32 and is supplied to the tuned amplifier 35.sub.1 and the first switching diode 35.sub.6 in the signal switching circuit 35.
On the other hand, the internal reference frequency signal outputted from the internal oscillator 33 passes through the buffer amplifier 34 and is supplied to the second switching diode 35.sub.7 in the signal switching circuit 35. Either the external reference frequency signal supplied to the first switching diode 35.sub.6 or the internal reference frequency signal supplied to the second switching diode 35.sub.7 is outputted via the coupling capacitor 35.sub.8 to the reference frequency signal output terminal 36.
A voltage of 5V is supplied from the high voltage power source terminal 37 to the tuned amplifier 35.sub.1 and is supplied to the collector of the first switching transistor 35.sub.4 via the collector resistor 35.sub.9. A voltage of 3V is supplied from the low voltage power source terminal 38 to the internal oscillator 33, the buffer amplifier 34 and the anode of the first switching diode 35.sub.6 The cathode of the second switching diode 35.sub.7 is connected to the ground via the bias resistor 35.sub.11.
The detecting circuit 35.sub.2 is connected to the tuned amplifier 35.sub.1 the smoothing circuit 35.sub.3 is connected to the detecting circuit 35.sub.2, and the base of the first switching transistor 35.sub.4 is connected to the smoothing circuit 35.sub.3. The collector of the first switching transistor 35.sub.4 is connected to the base of the second switching transistor 35.sub.5 via a resistor and is also connected via the inductor 35.sub.10 to the cathode of the first switching diode 35.sub.6 and the anode of the second switching diode 35.sub.7.
The internal oscillator 33 and the buffer amplifier 34 are connected to the ground via the second switching transistor 35.sub.5 which operates as a switch of the internal oscillator 33 and the buffer amplifier 34.
The operation of the conventional reference frequency signal switching circuit will be described as follows.
When the external reference frequency signal is not supplied to the external reference frequency signal input terminal 31, since the external reference frequency signal is not outputted from the tuned amplifier 35.sub.1 a DC voltage is not outputted from the detecting circuit 35.sub.2 and, therefore, a detection voltage is not outputted from the smoothing circuit 35.sub.3. Consequently, the first switching transistor 35.sub.4 is turned off. When the first switching transistor 35.sub.4 is turned off, the high voltage (5V) supplied to the high voltage power source terminal 37 is supplied via the collector resistor 35.sub.9 to the base of the second switching transistor 35.sub.5. Then the second switching transistor 35.sub.5 is turned on and the internal oscillator 33 and the buffer amplifier 34 are connected to the ground via the second switching transistor 35.sub.5. The voltage (3V) is supplied from the low voltage power source terminal to the internal oscillator 33 and the buffer amplifier 34, the internal oscillator 33 outputs the internal reference frequency signal, and the buffer amplifier 34 amplifies the signal. At this time, the signal is supplied via the collector resistor 35.sub.9 and the inductor 35.sub.10 to both the cathode of the first switching diode 35.sub.6 and the anode of the second switching diode 35.sub.7, so that the second switching diode 35.sub.7 is turned on. The internal reference frequency signal amplified by the buffer amplifier 34 is outputted from the reference frequency signal output terminal 36 via the second switching diode 35.sub.7 and the coupling capacitor 35.sub.8 On the other hand, the first switching diode 35.sub.6 enters an OFF state, so that the internal reference frequency signal appearing at the anode side of the second switching diode 35.sub.7 is not leaked to the external reference frequency signal terminal 31.
When the external reference frequency signal is supplied to the external reference frequency signal input terminal 31, the external reference frequency signal is amplified by the tuned amplifier 35.sub.1 detected by the detecting circuit 35.sub.2 smoothed by the smoothing circuit 35.sub.3, and a resultant is supplied as a detection voltage to the base of the first transistor 35.sub.4, thereby turning on the first switching transistor 35.sub.4. Then the collector of the first switching transistor 35.sub.4 approaches ground. Accordingly, the base of the second switching transistor 35.sub.5, the cathode of the first switching diode 35.sub.6 and the anode of the second switching diode 35.sub.7 approaches ground. The second switching transistor 35.sub.5 is therefore turned off and the voltage (3V) is not supplied from the low voltage power source terminal 38 to the internal oscillator 33 and the buffer amplifier 34. The internal oscillator 33 stops outputting the internal reference frequency signal and the buffer amplifier 34 also stops outputting the signal. The second switching diode 35.sub.7 is turned off, so that the internal oscillator 33 and the buffer amplifier 34 are isolated from the reference frequency signal output terminal 36. On the other hand, the low voltage (3V) supplied from the low voltage power source terminal 38 is supplied to the anode of the first switching diode 35.sub.6 and the cathode approaches ground. Consequently, a forward bias is applied and the first switching diode 35.sub.6 is turned on. The external reference frequency signal supplied to the external reference frequency signal input terminal 31 is outputted to the reference frequency signal output terminal 36.
As described above, according to the conventional reference frequency signal switching circuit, when the external reference frequency signal is not supplied to the external reference frequency signal input terminal 31, the internal reference frequency signal generated by the internal oscillator 33 is outputted. When the external reference frequency signal is supplied to the external reference frequency signal input terminal, the supplied external reference frequency signal is outputted as it is. In such a manner, the outputs of the reference frequency signals are automatically switched.
In the conventional reference frequency signal switching circuit, however, the supplied external reference frequency signal is outputted at the level corresponding to the inputted level. When the level of the external reference frequency signal fluctuates, the level of the reference frequency signal supplied from the reference frequency signal switching circuit to the transmitter also fluctuates, and a problem such that the transmitter does not operate normally occurs.